Electrolytic production of magnesium

ABSTRACT

Magnesium metal is produced by electrolysis of magnesium chloride employing a high surface area anode, for example, a porous anode to which hydrogen gas is fed. Hydrogen chloride is formed from the chloride ions at the anode, rather than chlorine gas; the process also has the advantage of operating at a lower voltage with a lower energy requirement than the conventional process in which chlorine gas is generated at the anode.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This Application is a Continuation of PCT/CA 00/00248, filed Mar.9, 2000, in which the United States of America was designated andelected and which remains pending in the International Phase until Sep.11, 2001.

TECHNICAL FIELD

[0002] This invention relates to production of magnesium byelectrolysis.

BACKGROUND ART

[0003] Conventional electrolytic production of magnesium from magnesiumchloride dissolved in a molten salt electrolyte in an electrolytic cellresults in formation of magnesium at the cathode and chlorine gas at thecathode. The molten salt electrolyte typically comprises one or morealkali metal or alkaline earth metal chlorides in which the magnesiumchloride is dissolved.

[0004] The production of chlorine as a by-product of the production ofmagnesium requires auxiliary equipment for recovery and storage of theby-product chlorine gas which typically is reacted with hydrogen gas toform hydrochloric acid. Electrolytic methods for producing magnesium aredescribed in U.S. Pat. Nos. 4,073,703; 4,192,724; 5,089,094 and5,665,220, the teachings of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

[0005] This invention seeks to provide a new electrolytic process forthe production of magnesium from magnesium chloride, in which hydrogenchloride is produced as the by-product.

[0006] This invention also seeks to provide a new electrolytic processfor the production of magnesium from magnesium chloride at a lowerenergy requirement.

[0007] In accordance with one aspect of the invention there is providedin a process for the electrolytic production of magnesium from magnesiumchloride in an electrolytic cell having an anode and a cathode, and inwhich magnesium is generated at the cathode, the improvement whereinhydrogen gas is fed to the anode and hydrogen chloride is formed in situat the anode.

[0008] In accordance with another aspect of the invention there isprovided a process for the electrolytic production of magnesiumcomprising: i) electrolysing magnesium chloride in a molten saltelectrolyte in an electrolysis cell having a cathode and an anode, withformation of magnesium metal at said cathode, ii) feeding hydrogen gasto said anode and reacting chloride ions at said anode with the hydrogengas to form hydrogen chloride, iii) recovering the magnesium metal fromsaid cell, and iv) recovering the hydrogen chloride from said cell.

[0009] In accordance with still another aspect of the invention there isprovided an electrolytic cell for production of magnesium metal frommagnesium chloride comprising: a) a cell for housing magnesium chloridein a molten salt electrolyte, said cell having a cathode and an anode,b) means for feeding hydrogen gas to said anode, c) means for recoveryfrom said cell of magnesium metal developed at said cathode, and d)means for recovery from said cell of hydrogen chloride developed at saidanode.

[0010] In accordance with yet another aspect of the invention there isprovided use of hydrogen in an electrolytic cell for the production ofmagnesium from magnesium chloride with production of by-product hydrogenchloride at the anode.

DESCRIPTION OF PREFERRED EMBODIMENT

[0011] In particular the anode is a high surface area anode, forexample, a porous anode in which case the hydrogen gas permeates thepores of the anode, such as by diffusion, or molten electrolytecontaining the magnesium chloride permeates the pores of the anode, toprovide the contact between the hydrogen gas and the chloride ions. Thehydrogen gas may be fed along a non-porous tube or conduit to the porousanode. If this tube or conduit is in contact with the bath it should notbe of a material which will function as an anode for the electrolysis.

[0012] As an alternative to a porous anode, any anode having a structurepermitting delivery of hydrogen to the cell bath at the anode may beemployed, for example, an anode having drilled channels forcommunication with a source of hydrogen gas. The requirement is that theanode structure delivers hydrogen gas to the cell bath at the anode, sothat chloride ions at the anode react with the hydrogen gas to formhydrogen chloride, rather than discharging as chlorine gas.

[0013] By way of example, suitable anodes may be of graphite, siliconcarbide or silicon nitride.

[0014] It has been found that introducing hydrogen at the anode in theelectrolytic cell for magnesium metal production results in a lowerenergy requirement for the cell, and the cell can be operated at a cellvoltage lower than the cell voltage of a corresponding cell having aconventional carbon or graphite anode, without hydrogen gas.

[0015] In addition it is found that hydrogen chloride is formed directlyat the anode by the reaction:

2Cl⁻+H₂(g)=2HCl_((g))+2e ⁻

[0016] where (g) indicates the gas phase.

[0017] Furthermore, the method has the advantage that this hydrogenchloride gas is produced with minimal, if any, production of chlorinegas.

[0018] In conventional cells in which chlorine gas is produced as theby-product, the anode is graphite, and at the high temperatures ofoperation some chlorinated hydrocarbons are produced by reaction betweenthe chlorine gas and the carbon anode, and this presents environmentalproblems. Eliminating production of chlorine gas in the presentinvention can be expected to alleviate these problems.

[0019] Table I below shows how the decomposition voltage of theelectrolysis decreases, with the process of the invention, as comparedwith the conventional process and how the minimum voltage required tomaintain energy balance changes. TABLE I Reaction E ° E_(adiab.)E_(adiab.) − E ° MgCl₂ → Mg + Cl₂ 2.50 3.60 1.1 MgCl₂ + H₂ → Mg + HCl1.46 2.74 1.28 Difference −1.04 −0.86 0.18

[0020] In Table I, E_(adiab) is the minimum voltage required to carryout the process, assuming 100% current efficiency and that the M_(g)Cl₂and H₂ are fed at room temperature.

[0021] In particular, Table I shows the calculated decomposition voltage(1000 K) and adiabatic voltage required to cover the energy requirementsof the process without heat losses.

[0022] Table I further shows that the decomposition voltage decreases by1.04V and that the overall energy requirement decreases by 0.86V. Thismeans that with HCl formation, another 0.18V per mole can be dissipatedin the cell without causing overheating. The decrease of 0.86Vtranslates to a reduction of about 25% less electricity consumption formagnesium production. With magnesium cells currently requiring anaverage of 12.5 MW-hr per tonne, and an average energy cost of 4 centsper KW-hrs, this translates to a savings of about $125 per tonne ofmagnesium produced in electrical consumption.

[0023] Another major cost saving comes from the fact that the cell isproducing HCl rather than chlorine, requiring no HCl synthesis plant.Chlorine treatment and handling as well as HCl synthesis can provide forfurther cost savings.

[0024] Environmental problems associated with chlorine gas productionare expected to be alleviated.

[0025] The hydrogen gas may be considered to form a hydrogen anode inthe cell, for discharge of the chloride ions. In such case an anodestructure is provided which, can be of any suitable material, forexample, graphite, silicon carbide or silicon nitride.

1. In a process for the electrolytic production of magnesium frommagnesium chloride dissolved in a molten salt electrolyte in anelectrolytic cell having an anode and a cathode, and in which magnesiumis generated at the cathode, the improvement wherein hydrogen gas is fedto the anode and hydrogen chloride is formed in situ at the anode.
 2. Aprocess according to claim 1, wherein the anode is a high surface areaanode.
 3. A process according to claim 1, wherein the anode is a porousanode and the hydrogen gas permeates the pores of the anode.
 4. Aprocess according to claim 1, wherein said anode is a porous anode andthe molten electrolyte permeates the pores of the porous anode.
 5. Aprocess according to claim 1, wherein the anode is of graphite, siliconcarbide or silicon nitride.
 6. A process, according to claim 1, for theelectrolytic production of magnesium comprising: i) electrolysingmagnesium chloride in a molten salt electrolyte in an electrolysis cellhaving a cathode and an anode, with formation of magnesium metal at saidcathode, ii) feeding hydrogen gas to said anode and reacting chlorideions at said anode with the hydrogen gas to form hydrogen chloride, iii)recovering the magnesium metal from said cell, and iv) recovering thehydrogen chloride from said cell.
 7. A process according to claim 6,wherein said cell is operated at a cell voltage lower than the cellvoltage of a corresponding cell having a carbon anode, without hydrogengas, in which chlorine gas is developed at the anode.
 8. A processaccording to claim 6, wherein said anode is a high surface area anode.9. A process according to claim 6, wherein said anode is a porous anodeand the hydrogen gas permeates from the pores of the anode into thecell.
 10. A process according to claim 6, wherein said anode is a porousanode and the molten electrolyte permeates the pores of the porousanode.
 11. A process according to claim 6, wherein said anode is ofgraphite, silicon carbide or silicon nitride
 12. An electrolytic cellfor production of magnesium metal from magnesium chloride comprising: a)a cell for housing magnesium chloride in a molten salt electrolyte, saidcell having a cathode and an anode, b) means for feeding hydrogen gas tosaid anode for production of hydrogen chloride at said anode, c) meansfor recovery from said cell of magnesium metal produced at said cathode,and d) means for recovery from said cell of hydrogen chloride developedat said anode.
 13. A cell according to claim 12, further including aconduit for delivery of hydrogen gas to said anode.
 14. A cell accordingto claim 12, wherein said anode is a high surface area anode.
 15. A cellaccording to claim 12, wherein said anode is a porous anode.
 16. A cellaccording to claim 12, wherein said anode is of graphite, siliconcarbide or silicon nitride
 17. A cell according to claim 15, whereinsaid means for feeding hydrogen to said anode, feeds the hydrogen gassuch that the hydrogen gas permeates from the pores of the porous anodeinto the cell.